The MPEG-2 standard describes an encoding method that results in substantial bandwidth reduction by a subjective lossy compression followed by a lossless compression. The encoded, compressed digital data is subsequently decompressed and decoded in an MPEG-2 compliant decoder. Video decoding in accordance with the MPEG-2 standard is described in detail in commonly assigned U.S. Pat. No. 5,576,765, entitled "Video Decoder" which is hereby incorporated herein by reference in its entirety.
Video decoders are typically embodied as general or special purpose processors and memory. For a conventional MPEG-2 decoder, two decoded reference frames are typically stored in memory at the same time. Thus, the cost of memory can often dominate the cost of the decode system. For example, an MPEG-2 video decoder might employ 2 MB or more of external memory, which generally comprises Dynamic Random Access Memory (DRAM). External memory is used for various data areas, or buffers such as frame buffers.
In practice, the MPEG-2 video decoder is typically limited to 2 MB of external memory in order to minimize cost of the end product. The decoder must perform all of its functions within this limitation. For example, of particular importance is enabling output for both the European market which utilizes the PAL standard of 576 video scan lines and the U.S. market which utilizes the NTSC standard of 480 video scan lines.
The MPEG-2 decompressed video data buffers, also called frame buffers, consume the largest part of external DRAM, therefore they are the prime candidate for memory reduction/compression. However, because the frame buffers contain final pixel display and MPEG reference data, any storage reduction technique must retain high video fidelity in the frame buffers.
Another obstacle faced in video compression/decompression is being able to transform pictures between different sized screens. For example, a motion picture screen is in 16:9 format, while a television screen is in 4:3 format. As a result, a method must be provided to convert between 16:9 and 4:3 form factors. This need is discussed in the above-incorporated, pending U.S. Application entitled "Multi-Format Reduced Memory MPEG-2 Compliant Decoder," which is described further herein.
In view of the above, and in order to establish commercial advantage, a technique is desired for implementing an enhanced and variable expansion filter within a multi-format reduced memory MPEG-2 compliant decoder. The filter is to be used for returning decimated data within the decoder to original format, e.g., for display. The present invention addresses this need.